In many producing, processing, transportation and storage processes, there are strict restrictions on the temperature, particularly the maximum temperature. For example, upper limit of temperature is required in the storage and transportation for many drugs in biomedical field, frozen food in food industry, constant-temperature water bath or oil bath in chemical engineering and storage environment for chemical materials and so on. For now, however, the real-time monitoring of the temperature of each product is not feasible economically and also not necessarily practical. Human vaccine, for instance, is a kind of special pharmaceutical products and requires strict “cold chain” in terms of transportation and storage, i.e., every step in the production, storage and issuing process for each vaccine must be imposed constantly into the required low temperature conditions in order to ensure its quality. At present, the incomplete cold chain (for example, the vaccine in the refrigerator is beyond the temperature limit in a certain period of time due to short-time power off) is an important reason for the deterioration of human vaccine. It's difficult to distinguish the deterioration of human vaccine caused by incomplete cold chain. The deteriorated vaccine not only fails to play an epidemic prevention role to the user, but also sometimes becomes a kind of harm, even a lethal threat. Although real-time monitoring of each vaccine's temperature change is significantly meaningful, the feasibility is unpleasant from the perspectives of economy and technology. The same problem also exists in the frozen food industry and other ones that require constant temperature or limited temperature range. To use a simple, reliable, economical and practical method to detect whether the individual product is beyond the temperature range and the exact exceeding degrees is of huge practical value.
The temperature indicating products which are currently used or reported are mostly electronic devices (CN101040175, CN1809851, CN200941054), mechanical device (CN2245765) and chemical solutions (CN 102336996 A). These technologies are available, but they have limited application with complex producing process and relatively high cost. Moreover, it's difficult to apply them to indicate the temperature of individual products.
A lot of polymer materials have the shape memory effect and are able to sense the changes in the environment and response in the form of morphology change (restore the initial state). Currently, the thermal induction is the most common induction for shape memory polymers and its principle is to employ the temperature sensitivity of the polymers to induce the spontaneous shape recovery. Thermal induced shape memory polymers stand out as the ideal choice to manufacture a large number of cheap temperature indicating products because of the shape memory effect and low cost.
Thermal induced shape memory polymers have the following characteristics:
(1) the predeformation: heating up the shaped shape memory polymer to a certain temperature, making the polymer transfer from glass state to rubber state or from solid phase to melting phase; and exerting deformation force to deform it and cooling down in the deformation state, making the polymer transfer from rubber state to glass state or from melting phase to solid phase as well as completely or partially maintain the deformation.
(2) the spontaneous shape recovery: when the deformed polymer is heated to the temperature above the initial temperature of glass transition or melting transition, the material can restore completely or partially to the original shape.